The present invention relates to a double servo system for use in positioning and contouring control in which both high resolving power and high speed are demanded. More particularly, the present invention pertains to an apparatus for controlling a piezo-electric actuator servo system which may be suitably applied to a precision stage in semiconductor lithography equipment or machine tools such as a lathe and a machining center in which extremely tight geometrical tolerances are demanded.
The present invention also relates to an apparatus for controlling a position control system employing a piezo-electric actuator which may be used in the field which is generally known as super-precision machining, for example, positioning in lithography equipment used in the manufacture of semiconductors, super-precision machine tools for producing laser optical parts, precision and fast-response piston lathes, etc.
There has been proposed an arrangement wherein a servo system which has high resolving power and a small stroke, for example, one which employs a piezo-electric actuator, is connected in series to a conventional long-stroke servo system which employs a motor or the like to realize a double servo system which has a long stroke and high resolving power, and this proposed arrangement has begun to be put into practical use.
Referring to FIG. 1, the reference numeral 1 denotes a motor, 2 a piezo-electric actuator, 3 and 4 moving members, 5 a slide mechanism, 6 a laser distance measuring equipment, 7 a corner cube, 8 a ball screw, 9 a tachometer generator, 10 a rotational angle detector, 11 a linear motor, 12 a linear scale, and 13 a detecting head.
There has heretofore been a system arrangement, for example, one which is shown in FIG. 1, as a double servo system which is formed by series-connecting a first servo system which feeds back at least position and speed and a second servo system which performs integral control by feeding back position. The illustrated system arrangement is formed by connecting together the first moving member 4 and the second moving member 3 through the slide mechanism 5 (schematically shown in the figure). For example, as shown in FIG. 1(a), the ball screw 8 is activated by the motor 1 to control the position of the moving member 4, and the moving member 3 is then finely moved by driving the piezoelectric actuator 2, thus effecting precision position control. In the case of this double servo system, the motor 1 is feedback-controlled on the basis of a motor rotational speed x.sub.M detected by the tachometer generator 9 and a motor rotational angle x.sub.M detected by the rotational angle detector 10, whereas the piezo-electric actuator 2 is feedback-controlled on the basis of the position x of the moving member 3 which is measured by a combination of the laser distance measuring equipment 6 and the corner cube 7. FIG. 1(b) shows a second example of the double servo system, in which the linear motor 11 is employed in place of the combination of the motor 1 and the ball screw 8 in the example shown in FIG. 1(a) and the combination of the linear scale 12 and the detecting head 13 is employed in place of the rotational angle detector 10 to measure the position x.sub.L of the moving member 4.
FIG. 2 is a block diagram showing the operation of the double servo system shown in FIG. 1(a), in which the portion surrounded with the chain line is the motor control system, while the portion surrounded with the one-dot chain line is the piezo-electric actuator control system. Since the piezo-electric actuator constitutes a proportionality system and has considerable hysteresis, it is general practice to subject the piezo-electric actuator to integral control, as illustrated. In this example, the piezo-electric actuator is feedback-controlled directly using a speed signal, whereas, in another example shown in FIG. 3, a circuit for obtaining the speed x.sub.L from the position x.sub.L is additionally provided.
As a still another example of the conventional double servo system, there is a system (Moriyama et al.: Super-Precision X-Y Moving Table Equipped with Piezo-Electric Actuator Fine Adjustment Mechanism, Journal of the Japan society of Precision Engineering Vol. 50, No. 4, 1984) such as that shown in FIG. 4. This double servo system is also composed of a coarse adjustment servo system and a fine adjustment servo system which are connected in series. More specifically, a first moving table 25 is mounted on a base 24, and a second moving table 26 is connected to the upper side of the first moving table 25. The coarse adjustment servo system employs DC motors 21, 22 as actuators to control the position of the first moving table 25 in the directions of X and Y by feeding back speed and position, whereas the fine adjustment servo system employs a piezo-electric element 23 as an actuator to constitute an integral control system which feedback-controls the position of the second moving table 26.
In this double servo system, the fine adjustment servo system is activated after the completion of positioning effected by the coarse adjustment servo system. This is because, if the coarse and fine adjustment servo systems are activated simultaneously, the two servo systems dynamically interfere with each other, resulting in an incorrect operation, as described later.
In the foregoing double servo systems shown in FIG. 1, the motor control system and the piezo-electric actuator control system may be considered to be independent of each other and therefore the two control systems may be activated simultaneously. In such a case, however, the following problems arise. Namely, the system shown in FIG. 1(a) needs the rotational speed detector 9, the rotational angle detector 10 and the processing circuit associated therewith in addition to the position detector for the moving member 3, and the system shown in FIG. 1(b) also needs the linear scale 12, the detecting head 13 and the processing circuit associated therewith in addition to the position detector for the moving member 3. Accordingly, the production cost increases correspondingly and, at the same time, the installation space increases unfavorably.
The object of control in the double servo system is to make the position of the moving member 3 equal to a desired value, and therefore provision of a detector for detecting the position x of the moving member 3 must suffice theoretically. On the basis of this idea, the control system shown in FIG. 5 employs the position x in place of the position x.sub.L employed in the control system shown in FIG. 3, as control data which is fed back to the motor control system.
The control system shown in FIG. 5 suffers, however, from the problem that the desired operation cannot be achieved because of the interference between the motor control system and the piezo-electric actuator control system. The reason for this may be explained with reference to FIG. 6 which is a modification of the block diagram shown in FIG. 5. As will be clear from FIG. 6, if the motor control system is regarded as a fundamental control system, the piezo-electric actuator control system is equivalent to a position loop having a D (derivative) action added thereto. However, the D action is taboo in servo systems. In particular, in the servo system having a completely closed loop which is handled in this application, the oscillation characteristics of the mechanical vibrating system are involved in the control loop, and therefore the D action is harmful to stability. Accordingly, it is difficult to impart practical characteristics to the double servo system having the arrangement shown in FIG. 5. The reason why the motor control system and the piezo-electric actuator control system in the systems shown in FIG. 1 can operate without interfering with each other is that the two control systems are controlled by their respective position and speed detectors.
Due to the above-described reasons, it is also impossible to simultaneously activate the coarse and fine adjustment servo systems in the conventional double servo system shown in FIG. 4. Therefore, the double servo system shown in FIG. 4 has the following problems:
(1) The time required to switch over the two servo systems from the coarse adjustment to the fine adjustment leads to a corresponding increase in the positioning time. (2) The double servo system cannot be used for contouring control in which the two servo systems must be activated simultaneously.
FIG. 7 shows a conventional control system which realizes a small displacement control system by the use of a feedback control system (see FIGS. 6 and 17 in Uchino "Piezo-Electric/Electrostrictive Actuators", Morikita Shuppan, p. 123, 1985). In this feedback control system, the applied voltage E is given as being the integrated value of the difference between the desired value e.sub.i and the position signal e.sub.0. Referring to FIG. 8, which is a block diagram of the feedback control system shown in FIG. 7, the block named the piezo-electric actuator shows the transfer function of the mechanical vibration system which consists of the actuator and mass. In the figure, .xi. is a parameter representative of the damping of the electrostrictive actuator.
FIG. 9 shows a prior art of a small displacement slide rest using a piezo-electric element (see the article in Nikkei Mechanical, Sept. 22, 1986). In this feedback control system also, a piezo-electric actuator is subjected to closed-loop control, but the feature of this prior art resides in that a notch filter is inserted in order to lower the gain near the natural frequency of the mechanical vibration system and enhance the stability of the closed loop.
In the conventional feedback control system shown in FIG. 7, however, the parameter .xi. that represents the damping of the electrostrictive actuator is considerably small in the case of ordinary elements, which results in a mechanical vibration system having inferior damping characteristics. FIG. 10 shows the results of examination of the response of the closed loop by a root locus plotted using the integral gain K.sub.3 as a parameter.
As shown in FIG. 10, the root which starts from the pole (.DELTA.) of the mechanicam vibration system becomes unstable as the gain K.sub.3 increases, and it reaches the stability limit at the limit gain K.sub.3c (the pole at this time is represented by .cndot.). Accordingly, in practice the gain K.sub.3 must be set at a considerably smaller value than K.sub.3c. Thus, in the conventional feedback control system shown in FIG. 7 it is impossible to set a large closed-loop gain in the case of a vibration system having inferior damping characteristics, and therefore this prior art suffers from the problems that the response of the closed loop is slow and the positioning accuracy cannot be increased.
In the feedback control system shown in FIG. 9, there is no problem of having the closed loop self-oscillating since the notch filter is inserted, but if the mechanical vibration system begins to oscillate at the natural frequency (the cut-off frequency of the notch filter) due to some reason, the notch filter does not work effectively at this frequency. Therefore, the oscillation must be damped only by the damping action of the mechanical vibration system itself, so that the oscillation continues for a disadvantageously long time. Further, the prior art shown in FIG. 9 suffers from the problem that the integrator may be offset in the low-frequency band since the gain does not reach infinity (.infin.) when the frequency f is 0.